A helicopter main rotor assembly is subjected to various aerodynamic, inertial, and centrifugal forces and moments during flight operations and to main rotor blade static droop when the helicopter powerplant is shut down. The main rotor assembly is designed to accommodate such forces and moments through the structural and functional characteristics of the various structural elements comprising the main rotor assembly. Of particular concern in designing a helicopter main rotor assembly are centrifugal loading (due to rotation of the rotor blades), torsional loading (due to pilot pitch control inputs), flapwise loading (due to out-of-plane blade motions, i.e., flapping), and edgewise or chordwise loading (due to in-plane blade motions, i.e., lead or lag).
Many prior art main rotor assemblies (older helicopters or those at the lower end of the cost spectrum) utilize mechanical mechanisms to react centrifugal, pitch, flapping, and/or lead-lag loads developed in the main rotor assembly. Blade attachment bolts have been utilized to transfer centrifugal loads from the main rotor blades to the main rotor hub structure. Bearings in the form of rolling element or elastomeric bearings have been utilized in mechanical hinges to react the pitch, flapping and lead-lag motions experienced by the main rotor blades. Mechanical linkages have been utilized to input pilot pitch changes to the main rotor blades.
Design and development efforts vis-a-vis main rotor assemblies have been directed to optimizing the functional characteristics thereof while concomitantly reducing weight and complexity to enhance the overall operational efficacy. The increased reliability, adaptability, reproducibility, and flexibility available from composite structures, due to advancements in composite materials and/or fabrication techniques, has led to the increased use of composite materials in helicopter main rotor assemblies. Individual composite structural elements may be designed and fabricated for main rotor assemblies to react a plurality of the main rotor assembly loading effects described hereinabove, thereby reducing the number of mechanical mechanisms required in the main rotor assembly to react centrifugal, pitch, flapwise, and/or chordwise loads and providing a concomitant reduction in the overall weight and complexity of the main rotor assembly.
The elimination of the offset flapping and lead-lag hinges in main rotor assemblies through the use of composite structural members has resulted in "bearingless" main rotor assemblies. The composite structural members of a bearingless main rotor assembly may be described as "flexbeams" or "crossbeams" due to the structural and/or functional characteristics of such composite structural members.
An exemplary composite crossbeam of a bearingless main rotor assembly is described in U.S. Pat. No. 4,746,272. The integral composite crossbeam described in the '272 patent is designed and fabricated to segregate the flapwise shear strain from the torsional shear strain to improve the fatigue performance of the crossbeam. An inboard flexure portion of each leg of the integral crossbeam is comprised of unidirectional composite fibers overwound or wrapped with a .+-.45.degree. composite wrap that forces torsional deflections outboard of the inboard flexure portion. The inboard flexure portion, therefore, reacts flapwise loads while an outboard flexure portion of each leg of the integral crossbeam reacts chordwise and torsion loads. The composite crossbeam provides an integral mechanical interface between opposed main rotor blades which accommodates centrifugal loading experienced by the opposed main rotor blades.
While much effort has been expended to optimize the structural and functional characteristics of composite flexbeams and crossbeams to enhance the overall efficacy of the main rotor assembly, minimal design and developmental work has been directed to optimizing the structural and functional characteristics of the main rotor assembly torque tube with respect to weight. What design and development work that has been done with respect to composite torque tubes has been driven primarily by the torsional stiffness requirements of the torque tube.
A torque tube is a hollow, elongated structural member that envelopes the main rotor assembly flexbeam or crossbeam and is connected at respective ends to the flexbeam (or crossbeam) and a main rotor blade. Each torque tube is operative to couple pilot-commanded pitch changes transmitted through a pitch input control device at the flexbeam end of the torque tube to the corresponding main rotor blade. This functional capability requires that torque tubes possess a certain torsional stiffness.
In addition to being designed and fabricated with a certain torsional stiffness to accommodate and react to pitch loading, torque tubes must also be designed and fabricated for fatigue strength and buckling strength to accommodate and react to flapwise and chordwise loading. The torque tubes must have sufficient fatigue strength to withstand the cyclic flapwise and chordwise loading experienced during normal main rotor assembly operations. Similarly, the torque tubes must have sufficient buckling strength to withstand the flapwise bending loads experienced during main rotor assembly startup which tend to induce buckling in the ventral and dorsal surfaces of the torque tube.
Prior art composite torque tubes have a structural configuration that includes an intermediate region having a constant wall thickness that provides the torsional stiffness and accommodates flapwise and chordwise bending loads, and thickened end regions that accommodate the coupling connections described hereinabove. Such prior art torque tubes have typically been formed with a .+-.45.degree. fiber matrix orientation (with respect to the main rotor blade pitch axis) to provide maximum torsional stiffness. While the .+-.45.degree. fiber matrix orientation satisfies the torsional stiffness requirements of the torque tube, this fiber matrix orientation is not optimal in providing a torque tube configuration that meets the torsional stiffness, buckling strength, and fatigue strength design requirements at minimal unit weight, i.e., the torque tube is overdesigned. The .+-.45.degree. fiber matrix orientation incurs a weight penalty in providing a greater than necessary wall thickness in the intermediate region.
There exists a need to provide a composite torque tube that is optimally fabricated to meet torsional stiffness, buckling strength, and fatigue strength design constraints at minimal unit weight. The optimally fabricated composite torque tube should also have a structural configuration that accommodates the loads and stresses experienced at the connection ends of the torque tube. Moreover, the fabrication technique for the composite torque tube should be consonant with the present level of composite materials manufacturing technology.